1. Field of the Invention
The present invention generally relates to fire detection and extinguishment systems for detecting and extinguishing fires in a protected space and, more particularly, to arrangements for improving sensitivity of fire detection and enhancing performance of fire extinguishment.
2. Description of the Related Art
Arrangements for detection of fires have been commercially available for many years. In particular, so-called smoke detectors using either ionization detectors to detect combustion products or photocell-type detectors to detect decrease in light intensity caused by opaque smoke particles have been effective and successful in providing early warning of fires in dwelling and other enclosed spaces.
However, these types of devices generally involve a fixed threshold or a weighted combination of detector output values in order to determine the existence of a fire. Often the threshold values are set high or the sensitivity is reduced in order to reduce the incidence of false alarms. Therefore, the installation location is often critical for proper performance of these devices. Specifically, installation is required to be in a location where smoke or combustion products are likely to accumulate, such as at horizontal ceilings enclosed by walls. In contrast, installations on walls or sloped surfaces often does not allow for accumulation of combustion products sufficient to actuate the alarm. Similarly, even when correctly located, air circulation currents (e.g. from an open window) may prevent proper operation of such sensors. In any event, the requirement of accumulation of combustion product slows the speed of detection since the fire must progress sufficiently to provide high levels of combustion products at the location of the sensor.
Known detectors also require significant amounts of maintenance in order to remain effective. For example, fouling of photocell detectors with dust, aerosols, moisture and the like can cause a decrease in measured level of light intensity and thus increased susceptibility to false alarms. On the other hand, fouling of ionization type detectors may reduce their sensitivity and prevent or delay detection of a fire.
Shipboard fires are particularly dangerous since escape from such fires is often limited by the architecture of the vessel. Additionally, auxiliary craft, such as life rafts and boats, which may be used to escape a fire, can expose personnel to other perils. Moreover, extinguishment of shipboard fires is complicated by the fact that the use of large quantities of water or other liquids may adversely affect the seaworthiness of the ship and may damage essential equipment. Therefore, it is important that shipboard fires be detected quickly to facilitate early extinguishment.
While residential type smoke alarms have been occasionally used in watercraft, they are generally less reliable than in fixed structures because of increased air circulation in moving vessels. Additionally, high temperatures and high humidity reduce the effectiveness of all types of fire sensors. Consequently, fire detection is complicated when enclosed areas of a vessel are subject to high humidity and periodic high levels of aerosols, such as from the galley and machinery, and to the high temperatures that are often encountered in mechanical areas of vessels such as engine rooms and auxiliary machinery spaces. Further, since most currently available detectors effectively require substantial progress of a fire before detection can occur, there is an increased likelihood of significant damage to the vessel before the fire can be extinguished. Any such damage to the vessel potentially increases risks to persons aboard the vessel.
The effectiveness of other types of detectors is also often impaired by shipboard conditions. For example, sensors which respond to rates of change of temperature often produce false alarms since vessels typically have many sources of heat which can cause localized temperature changes. By the same token, the illumination of enclosed spaces, typically by incandescent bulbs, can often cause infrared detectors to respond even though no fire is present. Sunlight reflected from water surfaces can also cause light-sensitive fire detectors to respond.
False alarm prevention is of particular concern aboard ships in connection with automatic fire extinguishment systems. Many fire extinguishment systems are charged with a fire extinguishing material and pressurized in order to function rapidly upon detection of a fire. Charging and pressurization of such systems often requires specialized equipment which may not be available aboard ship. A false alarm and actuation of such fire extinguishing systems may therefore leave a vessel vulnerable to a fire until the system can again be charged and pressurized. Further, to assure repeatable operability of such systems, operations such as draining or purging of the distribution system for the fire extinguishing fluids must be done after each actuation. Moreover, the actuation of such fire extinguishing systems may damage the vessel or its contents or reduce seaworthiness through contact with the fire extinguishing materials.
In summary, the trade-off between sensitivity of sensors and false alarms has kept the effectiveness of fire detection at a relatively low level, particularly for shipboard applications. This allows for unacceptable growth of potential fires prior to activation of automatic fire extinguishing equipment. Thus, there is a need for a system that provides early detection of shipboard fires, low probability of false alarms, and extinguishment of detected fires with minimal damage to the vessel or its contents.